Boating NZ : FREE TO READ April 2014
56 Boating New Zealand April 2014 tuck in of the forward waterlines. A well-powered, light displacement form with narrow waterlines forward and lots of chine sweep will end up with a fine vee and potentially narrow waterlines when underway, which could lead to stability issues. A heavier boat might suffer from a lack of lift in the bow and struggle to accelerate and get out of the hole – see diagrams. The issue of chine walking can be addressed by adding lifting strakes in the aft body. These serve as secondary chines to provide dynamic roll stability, and are placed on the hull bottom somewhere in the upper third of the deadrise. The designer’s software can output the running length, trim and chine immersion; a negative number indicates the chine is dry, that a strake is required, and where to put it. However they add drag at all speeds, in keeping with Rules One and Two. Spray rails – their location, size and number – is an equally debated topic. Their primary purpose is to shed spray as it climbs up the deadrise and therefore to reduce drag. The spray rails can be parallel to the centreline as buttock lines, parallel to the waterline or somewhere in between along a diagonal. I generally prefer the latter as it is usually perpendicular to the direction of the spray and sheds it best. That said, other orientations work better in some situations. The design considerations that apply to spray rails are similar to chines and strakes – too big and the ride will be harsh and possibly snappy in roll; too small, and they will be ineffective at shedding spray and the vessel may lack the benefit of roll damping underway. Generally spray rails shouldn’t extend too far aft into the immersed running body where they will create drag. Again, the designer’s software is a guide to where the wet bit starts at a given speed/trim combination, so we know where to stop them – see diagram. So intuition, design software to validate the intuition and real world feedback are required, but ultimately Rules Three and Five have to be observed – balancing all the requirements is tricky. SAVITSKY OUTPUT LIFTING STRAKES SPRAY RAILS Lifting strakes added to the after part of the hull can be used to combat chine walking and provide dynamic hull stability. They should be located somewhere in the upper third of the hull deadrise. The primary purpose of spray rails is to shed spray as it climbs up the deadrise, creating drag. They can be parallel to the centreline as above, parallel to the waterline or somewhere in between. ABOVE: The Rayglass Legend 2150’s spray rails are diagonal. Lift and tuck are clearer on port side. RIGHT: The Haines Hunter SF660 has buttock spray rails mostly vertical, parallel to the centreline. “Spray rails... primary purpose is to shed spray as it climbs up the deadrise and therefore to reduce drag.” REGULAR_Future concepts_April14.indd 56 19/03/2014 1:07:49 p.m.
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